Terrestrial Dynamo
The terrestrial dynamo is a physical phenomenon explaining the formation of the Terrestrial magnetic field, by the induction produced by the movements of alloys of iron and nickel in fusion in the liquid part of the core of the Earth.
Theory
See also: Magnetohydrodynamic
The phenomenon, although well included/understood in experiments, is not very well described yet by the theory. The important variations of the magnetic field and its inversions of polarity during geological time are the experimental modeling object, by taking for base the measurement of the variations of the terrestrial magnetic field. These measurements come from the observation of the magnetization of the marine sediments of the various oceanic basins.
It is thought that it is on the principle of the dynamo which the origin of the terrestrial magnetic field puts back. An electric current is created in a conducting loop moving in a magnetic field: it is the $dynamo effect. There is mechanical energy transformation into electromagnetic energy. Conversely, a current in a conducting loop produces a magnetic field. A current produces a field, a field produces a current, but without field and current initial, the $dynamo effect can start and be stabilized? (a fluctuation of current inducing a fluctuation of field which amplifies the current, etc…). It is the object of the recent experiments in the field where one studies the behavior of the turbulent flow (known as " of von Karman") of a conducting fluid (liquid sodium).
Experimentation of the phenomenon
Experiment VKS
Experiment VKS (Von Karman Sodium, ENS-Paris collaboration, ENS-Lyon and ECA-Saclay) was carried out at the ECA Cadarache, in France. It highlighted the spontaneous appearance of a magnetic field in a turbulent conducting fluid. The phenomenon reproduces the main features of the terrestrial dynamo with an order of magnitude respected. The threshold of appearance of the $dynamo effect corresponds to a Reynolds number magnetic (Rm) of 30. The Rm-Rmcritique transition is of supercritical type, within the meaning of the classification of the junctions.
It is by this experiment that it was also observed for the first time at the laboratory, in the very turbulent liquid sodium flow, of the inversions of the magnetic field similar to those of the terrestrial magnetic field. These experiments should make it possible to better include/understand the dynamics of the cosmic magnetic fields.
Associated scientific articles:
- observation of the $dynamo effect: " Generation off has Magnetic Field by Dynamo Action in has Turbulent Flow Liquid Sodium" off;.
- observation of the inversion of the field: " Magnetic field reversals in turbulent experimental year dynamo".
Experiment of Maryland
More colossal, with 3 tons of molten sodium and 1MW of mechanical energy injected; geometry with two encased spheres (thus closer to reality): but should be evacuated thermal energy: large oil ducts of cooling.-
Some more technical points: essence to be included/understood is to compare energy per unit of volume: EM = B ² /2 and EC. =V ². One wants to thus study EM = EC. .f (Re, Rm), where Re is the Reynolds number and Rm the Reynolds number magnetic. Turbulence being very tall, one works in Prandtl = very small Rm/Re (about 10^ (- 6)) thus very far from what the digital simulations (too different scales of time) can do: the time of diffusion of the momentum is very weak in front of that of the magnetic diffusion (in other words the fluid is far from viscous (the pressure and the temperature exploit bcp viscosity) whereas electric conductivity is less affected.
Evolution of the phenomenon during time
See also: Geomagnetism
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